Witchcraft is a genre of magic that is practiced by wiccans and some other people.
Witchcraft is a method that is used by people from different belief systems; it is not limited to Wicca alone. A good analogy of this is that witchcraft is to a religion such as Wicca what meditation is to a buddhist or what prayer is to a christian. Just as one does not need to be a buddhist in order to meditate, neither does one need to be a wiccan in order to practice witchcraft. Witchcraft is an act that one does, like praying or meditating, and is therefore not a belief system in itself.
In Wicca, witchcraft includes the use of crystals, runes, herbs, chanting, and other aspects used in spells and rituals.